The present invention relates to a method of making metal powder with low oxygen content, wherein the method includes atomizing molten metal by means of a pressurized gas which does not cause oxygenization, or is neutral or has reducing properties, such as nitrogen, argon or the like. More particularly, the present invention relates to improvements in such methods wherein molten metal is fed to an atomizer nozzle, and the solidified metal droplets are collected in a container underneath and cooled; whereby feeding, atomization, collecting and cooling is carried out in an enclosure under exclusion of oxygen.
Production of low oxygen content metal powder has recently become of increasing interest, and efforts along that line have increased accordingly. Generally, these known methods provide for atomization of molten metal in an air-tight container by means of an inert gas. The molten metal pours into and through an annular nozzle arrangement, or along a nozzle or nozzles, with longitudinal slots through which pressurized gas is directed at high speed and acts against the stream of molten metal. As a consequence, the metal stream is broken up, i.e. atomized, and metal droplets are produced which are collected and cooled in a container underneath the nozzle arrangement.
A water bath should not be used for cooling low oxygen metal powder, because of the oxygen contained in water; thus, the inert gas provides also the function of cooling the powder particles. The atomized metal droplets require some period of time for solidifying and cooling and the solidification process must be completed as the formed droplets fall and fly to the bottom of the container so that the atomizing container must be relatively high.
It was found, however, that even in the case of long flying trajectories and large vessel heights, such as 10 meters, the particles are still quite hot and they still have the tendency of sticking together. In other words, the powder as collected in the bottom of the container is not sufficiently loose and powdery, but is readily amenable to cake.
It has been tried to cool the container, particularly the bottom, from the outside. However, such cooling is not sufficient as the still relatively loose pile of powder (at least adjacent the cooled bottom) has very low thermal conductivity and that impedes cooling of the interior and upper portions of the powder heap. Consequently, only small quantities of powder can be produced here in one run before emptying the container and starting anew.
The discharge of powder during production is difficult as oxygen may very likely enter the atomizing chamber, etc. Thus, discontinuous production wherein atomization and powder removal alternate at a high rate was heretofore deemed necessary.